The invention relates to electrostatography and more particularly to a method and apparatus for applying a loading force to a roller used in image transfer.
In U.S. application Ser. No. 900,696 filed in the name of Tombs et al (the contents of which are incorporated herein by reference) there is disclosed an electrophotographic reproduction apparatus wherein a series of roller separation image forming stations are arranged so that a receiver sheet may be conveyed from station to station to receive color separation images in transfer so that the images are superposed onto each other to form a composite multicolor image. In the aforesaid application at each color separation station a compliant intermediate transfer drum or roller (ITR) has an electrically conductive core of for example aluminum, a relatively thick (1-20 mm) compliant blanket layer is around the core and a relatively thin (2 micrometersxe2x88x9230 micrometers) hard overcoat layer surrounds the blanket layer. The Young""s modulus of the blanket layer is preferably between 0.1 MPa and 10 MPa, and the blanket layer has a bulk volume electrical resistivity preferably between 107-1011 ohm-cm. The Young""s modulus of the overcoat layer is preferably greater than 100 MPa. The ITR forms a nip under pressure with a photoconductive (PC) drum. An electrical bias is impressed upon the ITR of suitable level and polarity to urge a developed toner image on the PC drum to transfer to the surface of the ITR. A receiver sheet is then moved into a second nip between the ITR and a paper transfer roller (PTR) in a timed or registered condition to receive the image in transfer from the ITR. An electrical bias of appropriate level and polarity is provided to the PTR to urge transfer of the toner image to the receiver sheet.
Heretofore, a PC drum and ITR were urged together by controlling the separation of their respective axes and establishing a predetermined interference in their respective radii of which interference is accommodated by the compliancy in the ITR blanket layer. However, this solution is not desirable because accommodation must be made when the machine is not producing prints to separate the PC drum from the ITR to avoid set forming in the ITR.
In the printing industry it is known as taught in U.S. Pat. No. 5,241,905 to use a pneumatic cylinder to apply a force to urge a blanket roller into engagement with a plate roller. A problem with this is that relatively costly controls are required to control the pressure in the pneumatic cylinder to maintain a constant loading force.
It is, therefore, an object of the invention to provide an improved method and apparatus for establishing an engagement between a pair of drums or rollers in an image transfer relationship that is relatively stable and insensitive to force changes in a system. While load may vary due to inherent xe2x80x9crun-outxe2x80x9d in the drums, the run out can be minimized through manufacture of the drums and the invention is directed to reducing force variations otherwise present or requiring increased expense to reduce.
It is a further object of the invention to provide a method and apparatus for ensuring that the engaged position is very repeatable between disengagement and reengagement.
In accordance with the first aspect of the invention there is provided a method for applying a predetermined load to a roller in a printer and/or copier apparatus comprising: supporting a first member relative to a frame, the first member including a driver element; advancing the driver element under a first force to establish a spring force in a spring; the driver member being stopped by a stop member to isolate any variability in the first force from affecting the spring force; providing a force receiving surface secured to said roller; engaging the force receiving surface with a second member, the second member being connected to the spring so as to provide a loading force to the roller.
In accordance with a second aspect of the invention there is provided an apparatus for applying a predetermined load to a roller in a printer and/or copier apparatus comprising: a driver element that is under a first force to establish a spring force in a spring; a stop member, the driver element being stopped by the stop member to isolate any variability in the first force from affecting the spring force; a force receiving surface secured to said roller; a second member that engages the force receiving surface, the second member being connected to the spring so as to provide a loading force to the roller.